1. Field Of The Invention
The present invention relates to a method and apparatus for detecting a pinhole. More specifically, the present invention relates to a method and apparatus for detecting a pinhole, wherein a high voltage is applied to a sealing sheet which is made of a synthetic resin and seals food, etc. or a closed container which is made of a glass or the like and used as an ampoule for containing medicine, and a pinhole being in existence on the sealing sheet or the closed container can be detected through detection of a discharge current which flows though the pinhole of the sealing sheet or the closed container.
2. Description of the Prior Art
A method and apparatus for detecting a pinhole of an insulation container in which contents such as food are sealed or contained has been proposed in, for example, Japanese Patent Publication No. 6998/1975. In such a method and apparatus, a high voltage is applied to the insulation container and a discharge current is detected to determine whether or not a pinhole is formed on the container. More specifically, the container is sandwiched by a pair of electrodes and the high voltage is applied between the pair of electrodes, and if and when spark discharge occurs between the pair of electrodes and the contents of the container, it is determined that a pinhole or pinholes are formed on the container by detecting a discharge current which flows between the pair of electrodes due to existence of the pinhole or pinholes.
Recently, in order to detect a pinhole of food which is sealed by a packaging sheet, a method as shown in FIG. 2 has been adopted. With reference to FIG. 2, food 2 such as ham which is vacuum-packaged by a packaging member 1 made of a synthetic resin such as polyethylene is put on a conveyor 3 which constitutes one electrode. A needle electrode 4 which constitutes another electrode is positioned above the food 2 with a slight interval or air gap such that a tip end of the needle electrode 4 can face the food 2. A high voltage is applied between the conveyor 3 and the needle electrode 4 by a control circuit 5.
In the case where a pinhole or pinholes are formed on the packaging member 1, since spark discharge occurs between the tip end of the needle electrode 4 and the food 2 and the discharge current flows between the needle electrode 4 and the conveyer 3, by sensing or detecting a discharge current by the control circuit 5, the pinhole or pinholes being in existence on the packaging member 1 can be detected.
In the above described prior art, in the case where a packaging member 1 is formed by a material not having heat resistance such as a thin polyethylene sheet, a pinhole might be formed on the packaging member 1 by corona discharge which takes place due to application of the high voltage.
Specifically, it was confirmed through an experimentation that in the case where the packaging member 1 is a polyethylene sheet having thickness of 40 .mu.m, if the voltage of 14 KV is applied between the both electrodes 3 and 4, a pinhole is formed on the polyethylene sheet for one or two seconds after the application of the voltage because the polyethylene sheet not having heat resistance is partially melted by heat generated by the corona discharge.
Therefore, in the case where the packaging member 1 is formed by the material not having heat resistance, in order to avoid such melting of the packaging member, the voltage being applied is to be lowered less than 10 KV. However, if the voltage being applied is lowered, detection accuracy is also decreased.
Especially, in the case of the food 2 such as salami, concave or convex is formed on a surface thereof as shown in FIG. 3, distance or air gap between the tip end of the needle electrode 4 and the packaging member 1 fluctuates when the food 2 is conveyed by the conveyer 3. Therefore, if the voltage is low, it becomes impossible to surely detect a pinhole or pinholes.
In addition, even if the packaging member is an ampoule made of a glass, it was found out through experience that a pinhole is also formed on the ampoule by the high voltage.
Therefore, in the prior art, in the case where the packaging member is formed by a material not having heat resistance, there is an antinomic problem that when the voltage to be applied is high a pinhole is undesirely formed on the packaging member, but when the voltage is lowered the detecting accuracy is reduced or it becomes impossible to detect a pinhole.